Although many synthetic and biological transformations involve oxocarbenium ion intermediates, details about the structure and reactivity of these highly reactive species remain unknown. For example, debate continues about the preferred conformation of the mannosyl cation, although drugs targeting this cation have been tested to treat genetic and viral diseases as well as cancer. In other cases, inconsistencies in the literature remain, such as the nature of anchimeric assistance in stereoselective glycosylation. This proposal builds upon our work during the last funding period that demonstrates the influence of heteroatom substituents on the conformations of oxocarbenium ions and thus the stereoselectivity of their reactions. We have devised an approach to solving the structure of the mannosyl cation, and we will gain insight into the nature of anchimeric assistance. We have also devised new stereoselective synthetic methods based upon oxocarbenium and peroxocarbenium ion intermediates. This proposal will have considerable implications for public health. For example, our studies on the three- dimensional structures of carbohydrate-derived intermediates will impact the design of drugs for the treatment of diabetes, hepatitis, HIV, and cancer. In addition, our studies on the structure and reactivity of these intermediates will enable chemists to prepare those drugs more efficiently.